Also worthy of mention are terahertz gas lasers which are typically excited using a CO\(_2\)  pump laser and have been a source of CW coherent terahertz radiation above 0.3  for many years [1]}. They are based on rotational transitions of molecules meaning that they are not continuously tunable. Gas lasers can reach up to 1 mW of THz power for 1 W of pump power. Recently it was demonstrated that the combination of a quantum cascade laser with a nitrous oxide molecular gas laser, can offer a widely tunable THz source [2]}. Finally, we mention two other terahertz sources, the first is the backward wave oscillator (BWO) which produces narrow linewidth, tunable terahertz radiation with high output power. In recent times, manufacture of these devices has all but ceased as AMC sources have become higher power and provide a more compact and versatile solution. Finally we mention the free-electron laser (FEL), which is a facility-scale radiation source. FELs can produce very high (several watts) output power in the terahertz range. A summary of the relevant properties of various terahertz sources is shown in Table REF .
